Various apparatus and methods have been used to hydraulically separate slurries containing solids of different settling rates into several fractions. For example, U.S. Pat. No. 4,539,103, Sept. 3, 1985, granted to applicant, discloses a hydraulic separator suitable, for example, for the sizing of aqueous slurries containing solids differing in size and settling velocities, such as sand. The apparatus is characterized by a tank into which feed slurry is iintroduced tangentially to provide swirling of the material in the tank. The lower portion of the tank is provided with an annular barrier in the form of a perforated constriction plate and the upper end of the tank forms an overflow weir. Deflecting means in the form of a cone is mounted upon the constriction plate and serves to deflect solids moving downwardly toward the space immediately above the annular constriction plate. Means is also provided for introducing elutriation water into the space below the constriction plate. An underflow is removed from the region immediately above the constriction plate. Such apparatus functions efficiently for sizing and separating operations where overflow and underflow fractions are desired.
U.S. Pat. No. 3,308,951, Mar. 14, 1967, discloses apparatus which is stated to be capable of separating hydrous slurry into overflow, underflow and middling fractions. FIG. 2 of the patent discloses a tank structure made into aligned upper and lower parts, the lower part being provided with two constriction plates. Feed slurry is introduced into the upper tank part, and an overflow of fine product is removed from the upper part. The coarser solids progress downwardly through a cylindrical column to the lower part, where two fractions accumulate above the constriction plates, one consisting of coarse solids, and the other of solids of intermediate size. Siphoning pipes are used to remove the fractions accumulating above the constriction plates. Such apparatus and its method of operation, are subject to a number of operating disadvantages. The removal of solids by siphoning pipes tends to be critical with respect to control, particularly when the solids content of the feed slurry is subject to variations or the rate with which the slurry is introduced is subject to changes. Also siphoning pipes are subject to clogging.
U.S. Pat. No. 1,449,603 issued Mar. 27, 1923 discloses an apparatus and method for separating the particles of a slurry into three fractions having particles differing in settling velocities, but of the same particle size. Three concentric tank sections 5, 7 and 13 are employed to provide draw-off of particles differing in settling velocities. The heavier particles progress to the lower end of each section 7 and 13 and the lightest particles discharge as an overflow from the upper section 13. The feed slurry is first processed to reduce the solid particles to a common size and then introduced into the lower most tank section 5. From thence material progresses to the upper tank section 13 from which the particles of lowest specific gravity are removed in an overflow. Each tank section 7 and 13 at its lower end has an annular portion with holes 9 and 13 that receive separated particles of greatest specific gravity that are caused to settle in the main quiescent body of the corresponding chamber of the tank section. Water is introduced upwardly through these holes to maintain upward progression of lighter particles to the upper end of the tank section. The separating action takes place in the quiescent chambers A, B and E.
In general there is a demand for a high capacity apparatus and method capable of operating upon various feed slurries, which provides for the withdrawal of an intermediate or middling fraction, which is lacking in criticality and other operating difficulties.